The present invention relates to a circuit arrangement, including a firing element for a seat belt tightener, and a belt operation sensor, which can be connected via a common wiring pair to a control unit that evaluates the belt operation sensor and diagnoses the firing element.
A circuit arrangement is described in German Published Patent Application No. 195 05 334. This known circuit arrangement has a circuit element that characterizes the working position of the seat belt. In this manner, detecting the working position of the seat belt is made possible by an electronic device. Thus the goal is to detect whether the belt is inserted properly in the buckle, when a person is situated in the seat of the vehicle. If appropriate, when a seat belt has not been inserted properly, a warning signal is emitted. In accordance with this publication, both the belt operating sensor as well as the firing element for the belt tightener are connected to a control device by a common symmetrical wiring pair. As a result of the fact that the belt operating sensor and the firing element are connected to the control unit by their own lines, the risk is reduced that the many lines can result in the coupling of many interference signals. In addition, the complexity of the cable tree and the number of necessary plug-in contacts are reduced as a result. According to German Published Patent Application No. 195 05 334, the firing element for the belt tightener and a buckle switch which is used as a belt operating sensor are connected in parallel to a common wiring pair. Furthermore, a resistor is also connected in series to the buckle switch and a further resistor is connected thereto in parallel.
The present invention is based on the object of indicating a circuit arrangement of the type mentioned above, which can permit the belt operation to be clearly ascertained and the state of the firing element to be reliably diagnosed.
According to the present invention, the firing element, the belt operating sensor, and a first high-resistance resistor are connected in parallel, the parallel circuit is connected in series to a second low-resistance resistor, and the series circuit composed of the second resistor and the parallel circuit is connected to the two lines. The belt operating sensor is advantageously a mechanically actuatable switch in a buckle, which is opened if a belt tongue is snapped into the buckle, and which is otherwise closed. In this context, the circuit arrangement is a very inexpensive one, having only two ohmic resistors in addition to the firing element and the buckle switch.
According to the present invention, the circuit arrangement is characterized by the fact that a high-resistance resistor is connected in parallel to the firing element, that an electrically controllable switch is connected in series to the parallel circuit composed of the firing element and the resistor and this series circuit is connected to the two lines, and that an element that is sensitive to a magnetic field is present, which generates for the switch a control voltage, which is a function of a magnetic field that is influenced by a belt tongue that is inserted into the buckle. One advantage of this circuit arrangement lies in the fact that it does not have, as a belt operating sensor, a mechanical switch, which is sensitive to contamination, but rather that it is completely wear-free, insensitive to contamination, and connected in a contactless manner, as a result of using an element that is sensitive to a magnetic field.
According to the present invention, it is expedient that the element sensitive to the magnetic field is a Hall element.
According to one refinement of the circuit arrangement, between the two lines, a capacitor is connected in series to the parallel circuit composed of the firing element and the resistor, the capacitor receiving from the control unit a charging current, the capacitor cyclically charging and discharging, and its discharging current flowing through the element that is sensitive to the magnetic field, so that, in this phase, the magnetic-field-sensitive element generates a control voltage that closes the switch. During the charging process that follows the discharge of the capacitor, the switch remains open, due to an insufficient control voltage of the magnetic-field-sensitive element. The control unit can now detect the belt operation, from the pulse duty factor of opening and closing the switch, which, when the belt tongue is snapped into the buckle, is distinguished from the pulse duty factor when the belt is not snapped in. Or the control unit can detect the belt operation from the current on the lines, the current being greater when the belt tongue is snapped in than when it is not, the control unit measuring the current during the open phases of the switch. The diagnosis of the firing element is conducted by the control unit during the closed phases of the switch.